The invention concerns a process for the production of spherical fuel and/or fertile particles with a high throughput by casting uranium and/or thorium and/or plutonium compound containing solutions from a vibrating nozzle through an ammonia containing gas zone into an ammoniacal precipitation bath with subsequent washing, drying and sintering or melting of the formed and solidifed drops.
All fuel elements for high temperature reactors contain the fuel and fertile material in the form of coated particles. These coated particles consist of uniform spherical particles of the oxide or carbide of uranium and/or thorium and/or plutonium which are jacketed by layers of pyrolytic carbon and silicon carbide in order to retain the fission products.
At the present time essentially the following types of particles are of significance: uranium oxide (UO.sub.2), uranium carbide (UC.sub.2), thorium oxide (ThO.sub.2) and uranium-thorium oxide mixed crystals (U,Th)O.sub.2. According to the requirement the particles are between 200 and 600.mu., the density should be as high as possible.
For the feed-breed-fuel cycle preferred for the high temperature power reactors, there are produced both thorium oxide-kernels with a diameter of about 500.mu. and also uranium carbide-fissile kernels with a diameter of about 200.mu..
In the further progress of the production of fuel elements these fissile particles and fertile particles were provided with coatings of pyrolytic carbon and silicon carbide which serve to retain the fission products in the operation of the reactor. Thereby the fission gas pressure occurring in the particle interiors, the strains formed by neutron emission and the stresses caused by temperature changes must be intercepted by these layers. This is best possible if these layers are precise spherical shells and the kernels have an exact spherical shape.
There are known various processes employed for the production of such particles. Besides powder metallurgical granulation processes which are not capable to yield completely exact spherical shapes wet chemical processes were developed which for industrial and economical reasons are preferred to dry processes. In practically all wet chemical processes an aqueous solution or a sol is produced from a uranium, thorium and/or plutonium compound. In the case of carbide production this liquid contains finely divided carbon in dispersion, Then drops are formed from the solution or sol which are as spherical as possible and which are solidified with retention of the spherical shape. The solidified particles are, in case there are present additives or waste products e.g. ammonium nitrate which are to be removed, washed, then dried, calcined and, in the case of oxide particles, sintered. For the production of carbide the particles after the calcination are brought to reaction and either sintered at 1700.degree.-2000.degree. C. or melted at about 2500.degree. C.
One of these known processes is the so-called sol-gel method, according to which an aqueous thorium oxide or thorium-uranium oxide sol is produced and is dropped into an organic solvent, e.g., 2-ethyl hexanol, which is not miscible with water. The solvent used has a solubility for water of several volume percent. The viscous sol drops are caused to get by removal of water and solified (Kleinsteuber U.S. Pat. No. 3,171,715 and Clinton U.S. Pat. No. 3,290,122).
According to another process the aqueous solution of compounds of the materials from which the spherical particles should be formed is mixed with a resin, preferably methyl cellulose, which imparts a very strong increase in viscosity in dropping into alkaline solution, so that the spherical shape of the drops remains in the slow penetration of the base into the interior of the drops and the precipitation and hardening associated therewith (Brambilla German Pat. No. 1,212,841). The addition of monohydric or polyhydric alcohols to this viscous drop solution improves the surface tension and therewith the formation of the sphere (Neri German Pat. No. 1,671,051).
From Fleichhauer German Auslegeschrift No. 1,542,346 there is known a process in which an emulsion is formed from a metal salt solution with an oil, this emulsion is treated with polyvinyl alcohol (PVA) and dropped into the ammonia solution, whereby the drops are solidified very quickly. Oil and PVA are washed out of the solidified particles and these particles dried and finished.
From Nickel et al. German Auslegeschrift No. 1,960,289 there is known a process for the production of spherical particles from uranium dioxide in which solid hexamethylene tetramine (hexa) is dissolved in an aqueous solution of uranyl nitrate and urea and this cooled concentrated solution dropped into hot oil. The spherical particles solidified thereby the ammonia split out from the hexa and washed, dried and sintered.
A further process is known from German Offenlegungsschrift No. 2,147,472 in which the metal salt solution or the sol is dropped into a ketone phase which stands over an aqueous phase. In German Offenlegungsschrift No. 2,323,072 it is additionally disclosed that for improving the particle density and the yield it is advantageous to mix the metal salt solution with ammonium nitrate and urea before dropped into the ketone phase.
From both of the last named German OS and Graham et al. U.S. Pat. No. 3,204,934 and Flack U.S. Pat. No. 3,617,584 it is known to subject the liquid jet to a vibration in the dropping in order to produce uniform drops with increased throughput. All of the previously mentioned processes have the disadvantage that the increase in throughput which would be possible by increase of the frequency of drops is limited either because of the adverse hindering of the drops in the liquid in which they are formed on leaving the dropping nozzle or through the necessarily high viscosity of the dropping liquid. Besides several of the processes mentioned have the disadvantage that polymeric organic materials are used as additives for the dropping solution which either even at slight changes of the pH or through aging in the solution change their condition and thereby change the viscosity of the solution so that it is difficult in large scale production always to produce exactly uniform dropping solutions and to work them up, whereby, particularly in the drying and the further heating of the particles formed there occur high amount of scrap due to breaking and disintegration.
Therefore, it has been proposed in Huschka German published patent application No. P 2,459,445 and related Huschka U.S. application Ser. No. 638,662 filed Dec. 8, 1975 to make possible an increase in throughput in the drop formation from uranium and/or thorium solutions treated with PVA by flowing the vibrating or oscillating liquid stream after leaving the nozzle through a first gaseous falling zone free of ammonia and in which the exactly spherical liquid drops next fall through a second gaseous falling zone containing ammonia and thereby (also at a very high frequency of drops) are so well flowed around on all sides that they are uniformly solidified and upon entering an aqueous ammonia solution located in the falling column below the second gaseous falling zone are not deformed. However, also with this process there can only be obtained a maximum of 80,000 drops per minute per nozzle, depending on the viscosity of the solution. The entire disclosure of the Huschka United States application is hereby incorporated by reference and relied upon.